[linux-lvm] hmmm, curious...

[Holger Grothe]

On Thu, Jan 31, 2002 at 09:11:13AM +0800, Colin Coe wrote:
> 
> Hi all, I have come accross a strange problem while doing some scripting
> with lvdisplay.  When I use lvdisplay piped to awk in the first example
> (noting that I have 'hard-wired' one field in awk), a second line is printed
> containing all hard-wired strings, including tabs.  But if I encase the
> lvdisplay in backticks and use echo then piped to awk the problem does not
> exist.
> 
The whole "problem" is no problem and has absolutely nothing to do with lvm.
It's simply shell programming.

> [root at linux-lvm bin]# lvdisplay -c /dev/rootvg/bbs | awk -F: '{print $1
> "\t\t" $9 "\t" $9 "\t" "2" "\t" $4 }'
> /dev/rootvg/bbs         16      16      2       1
>                                 2
>
Works as designed :-). The output of lvdisplay -c <logvol> has two
lines. One line with the informations and one empty line. The empty
line contains no fourth or nineth field ==> Only "fixed" things are 
printed for the second line. (awk is primarily a line orientated tool).
  
> [root at linux-lvm bin]# echo `lvdisplay -c /dev/rootvg/bbs` | awk -F: '{print
> $1 "\t\t" $9 "\t" $9 "\t" "2" "\t" $4 }'
> /dev/rootvg/bbs         16      16      2       1
>
Assuming you are using bash (from man bash):

              `command`

       Bash  performs  the  expansion  by  executing  command and
       replacing the command substitution with the standard  out­
       put  of  the  command, with any trailing newlines deleted.
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
So, it's a feature of the shell.       
 
> I have tried using sed to strip out newline characters, but this has not
> helped.  lvdisplay -c /dev/rootvg/bbs | awk -F: '{print $1 "\t\t" $9 "\t" $9
> "\t" "2" "\t" $4 }' | sed "s/\n/ /g"
> 
> I have a work around so it is not a major issue, but I thought I'd raise it
> anyway.
> 
If you wanna use only the first line of lvdisplay just use only the first
line, e.g. use head in pipe.

lvdisplay -c <logvol>| head -1| awk ...

Holger
-- 
Holger Grothe  (Email: grothe at mathematik.tu-darmstadt.de)
Fachbereich Mathematik, TU Darmstadt